Why do we see eddy patterns above a radiator

The air around a hot radiator soon acquires heat. We explain this observation from the minus oneth law of thermodynamics (see Section 1.1), since heat travels from hot to cold. 

The density of a gas depends quite strongly on its temperature, so hot air has a smaller density than does cold air colder air is more dense than hot air. From everyday experience, we know that something is dense if it tries to drop, which is why a stone drops to the bottom of a pond and a coin sinks to the bottom of a pan of water. This relative motion occurs because both the stone and the coin have higher densities than does water, so they drop. Similarly, we are more dense than air and will drop if we fall off a roof. 

Just like the coin in water, cold air sinks because it is denser than warmer air. We sometimes see this situation stated as warm air displaces the cold air, which subsequently takes its place. Alternatively, we say warm air rises , which explains why we place our clothes above a radiator to dry them, rather than below it. 

So we see swirling eddy (or convective ) patterns above a radiator because the density of air is a function of temperature. If all the air had the same temperature, then no such difference in density would exist, and hence we would see no refraction and no eddy currents - which is the case in the summer when the radiator is switched off. Then again, we can sometimes see a heat haze above a hot road, which is caused by exactly the same phenomenon. 

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